This is a Phase II, multicenter, open label study of BG9588 (Biogen's humanized anti-CD40L monoclonal antibody) in subjects with SLE GN. The primary objective is to determine if BG9588 can reduce proteinuria by greater than 50% from baseline at two consecutive visits between Day 57 and Day 141, without worsening of renal function, in subjects with WHO Class III, IV or mixed membranous and proliferative SLE GN. The secondary objectives are to determine the safety and pharmacokinetics of BG9588 and its effects on serum C3 complement levels, hematuria, urinary cellular casts, anti-dsDNA titers, SELENA-SLEDAI and SF36TM.